import numpy as np
import matplotlib.pyplot as plt
plt.title('what',fontsize=18)
plt.xlim(1,5)
plt.ylim(2,6)
x = np.linspace(-3,3,100)
y = np.sqrt(9 - x**2)
plt.plot(x,y,x,-y)#先画上半圆，再画下半圆
plt.axis('equal')#设置x，y轴单位长度相等
plt.show()